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This makes the event serialization code handle TouchEvents. I plan on
doing a cleanup pass to make MouseEvent serialization use the PointerDetails
struct traits.

BUG=865781
TEST=covered by tests

Change-Id: I190ac58a3066c10bc08cd083702bc15232d4141d
Reviewed-on: https://chromium-review.googlesource.com/c/1265160
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597383}